In recent years, with the popularization and development of mobile terminals, new communication systems continuously pursue higher transmission rate and greater channel capacity. In 4G communication systems (Long Term Evolution (LTE) and evolved LTE-A, Worldwide Interoperability for Microwave Access (WiMAX) systems, etc.), a Multi-Input Multi-Output (MIMO) antenna technology becomes a core feature for improving data rate. It generally refers to that a plurality of antennas are deployed at a receiving end and a transmitting end of a wireless communication system and a plurality of parallel transmission channels are formed in the same space such that a plurality of data streams are transmitted in parallel by using these independent channels, so as to increase system capacity and improve spectrum utilization rate.
For an MIMO communication system, under the situation that a plurality of antennas are arranged closely in a space, received signals of the antennas therebetween have a correlation. The greater the correlation is, the lower the independence of each signal channel is, and the more obvious the deterioration influence on the overall transmission performance of the system is. Therefore, to effectively reduce the correlation between the antennas in the MIMO system and improve the isolation of the antenna is a key technical point for realizing high-speed data transmission of the MIMO system. With the further evolution of the technology, in order to support higher transmission rate, the latest LTE-Advanced standard (3GPP Release 12) has already supported a 4×4 MIMI technology, that is, four antennas are deployed on both a transmitting end and a receiving end, i.e., a base station and a mobile phone terminal, and the four antennas simultaneously work and there are not the primary and secondary points. It is required that each antenna has balanced radio-frequency and electromagnetic performance, and a lower correlation and a higher isolation are kept between all antennas.
On a base station side, since there is no strict requirement on the space occupied by base station antennas, the correlation between the antennas can be reduced by increasing the spacing between the antennas or by means of orthogonal polarization between the antennas. However, on a terminal side, especially on a mobilephone terminal, due to restriction of physical size, it is a very great technical challenge to deploy a plurality of antennas and keep lower correlation and higher isolation between the antennas. Terminal miniaturization demands prevent the isolation from being improved by increasing the spacing between the antennas, and small antenna radiation of the terminal usually has not an obvious polarization trend and thus it is very difficult to improve the isolation of the terminal antennas by means of simple orthogonal polarization. Thus, at a current stage, the terminal generally is provided with two antennas only, i.e., a main antenna and an auxiliary antenna, wherein, the main antenna is used independently for receiving and transmitting radio communication signals and the auxiliary antenna may work in an MIMO receiving mode to improve signal data transmission rate.
Traditional methods for improving isolation of terminal antennas generally are divided into three types: adopting different types of antenna combinations and different placing positions; increasing floor parasitic metal conductors or parasitic slit structures to change antenna mutual-coupling; and increasing decoupling lines/balancing lines/decoupling networks between antennas. Wherein the method of the first type is greatly restricted by intrinsic physical size of the terminal and it is difficult to apply in practice; and for the methods of the second and third types, the decoupling bandwidth is relatively very narrow, at present it is found that the effect is better mainly for above-2 GHz high frequency bands, such as LTE Band 7 (2500-2690 MHz), LTE Band 40 (2300-2400 MHz), etc. However, for LTE 700 MHz low frequency bands, such as LTE Band 12 (698-746 MHz), LTE Band 13 (746-787 MHz) and LTE Band 17 (704-746 MHz), the decoupling effect is not good and it is difficult to satisfy wide frequency band feature which is actually needed. At present, as considered by the academic community of antennas, the MIMO system requires the multi-antenna index of the terminal to be that the efficiency of a single antenna is above 40% and the isolation of any two antennas is above 15 dB. Therefore, when four LTE low frequency band antennas are deployed in a space where a handheld terminal is seriously limited, deploy, to guarantee higher isolation which needs to guarantee antenna efficiency and reduce coupling between the antennas becomes a key difficulty in 4×4 MIMO antenna design of the terminal.